The present invention relates generally to thin endless bands as well as a method and apparatus for their production. More particularly, the present invention concerns cold rolled endless metal bands, the method of producing those endless metal bands, and apparatus for producing those metal bands.
As used herein, a thin sheet refers to an elastic sheet having sufficient thickness to be resilient when bending stresses are applied, having uniform thickness which does not vary more than 10% above or below the average thickness, having a length along the neutral axis of bending which length is at least twenty times greater than the average thickness and having a width normal to the plane of the neutral axis which is at least greater than the thickness. The actual width of the sheet is irrelevant and need not be uniform. Generally speaking, the average thickness of the sheets must exceed 0.0001-0.0003 inches. While sheets with smaller thicknesses of common sheet materials exhibit raw tensile strength, those sheets cease to exhibit useful structural properties such as bending resilience. And, the average thickness of thin sheets is typically less than 0.10 inches since thicker plates exhibit bending stiffness which is too large.
A ribbon as used herein is a special case of the thin sheet. A ribbon is a thin sheet with a uniform width of at least twenty times the sheet thickness and has parallel side edges. Typically, a ribbon also has smooth surfaces, uniform surface finish, and uniform edge structure.
As used herein, an endless band will be considered to be a ribbon having no ends and forming a closed loop. In the past, a great many different processes have been proposed and employed for making thin sheets and ribbons from continuous materials. However, no process or apparatus has been found which can make an endless metal band without starting from a ribbon. Moreover, few, if any, of the processes for making thin sheets and ribbons have proven to be practical in terms of making highly uniform thin metal sheets and ribbons from materials having high moduli of elasticity, such as spring metals.
Extremely high pressures are required to extrude hot or liquid metals into thin sheets with thicknesses of one hundred-thousandths of an inch (0.100) or less. Moreover, currently available die materials have extremely short useful lives when hot or liquid metals are extruded therethrough. Accordingly, extrusion processes using presently available die materials have proven to be impractical.
The principal process permitting fabrication of thin sheets and ribbons is rolling which may be done with either hot or cold material. In the rolling process, the material passes through the nip of two hard, smooth rollers and is elongated by the reduction in thickness. Since the rollers can be given highly uniform surface finishes, thin sheets and ribbons with reasonably uniform properties can be made.
To date, there are five different types of rolling mill arrangements used in producing metal sheets: two-high mills, three-high mills, four-high mills, cluster mills, and Sendzimir mills. In the two-high, three-high, and four-high mill arrangements, all rollers are arranged with parallel coplanar axes. The cluster mill, on the other hand, has a pair of work rolls with parallel coplanar axes but each work roll is backed by a pair of larger backing rolls having parallel axes which lie in a plane perpendicular to the plane containing axes of the work rolls.
Turning now to the Sendzimir mill, each of a pair of work rolls having parallel coplanar axes is backed by a pair of larger first intermediate rolls. These first intermediate rolls are in turn backed by three drive rolls which themselves are supported by four back-up rolls. Generally speaking, the Sendzimir mill has proven to be superior for fabrication of thin sheets and ribbons since adequate rolling pressures can be maintained without significant bending of the work roller.
To make a band, the past practice has involved forming a ribbon and then fabricating the endless band from the ribbon. Ends of the ribbon were connected by welding, brazing, and similar methods. Moreover, the mating ends of the ribbon were butt welded or attached with a diagonal scarf joint. Endless metal bands fabricated in this foregoing fashion exhibit poor fatigue strength when subjected to repetitive cyclic bending forces. In this connection, it can be seen that the weld, whether it be at a butt joint or a scarf joint, creates a structurally dissimilar area where stress concentrations occur during each bending cycle.
Another problem with known metal bands is their propensity to have zipper-like crack failures which extend perpendicularly to the parallel edges of the band. These cracks typically form when the band is subjected to repetitive cyclic bending.
Endless metal bands which are free from the structural weaknesses caused by the presence of welds and the tendency to propagate cracks normal to the parallel edges have potentially wide-ranging uses. For example, such endless metal bands may be useful in speed-changing devices, as replacements for conventional drive belts, and other power transmitting flexible devices. Accordingly, it will be seen that the need exists for endless metal bands having properties of the type described as well as for a method and apparatus for producing those bands.